I agree that with Harriet's heart being given to Robert Martin, the problem of a secret being kept from Mr. Knightley disappears. I will add that it is gloriously helpful to all that Harriet's momentary embarrassment disappears practically in the same instant! She is not even mortified that she once imagined herself in love with Mr. Knightley now -- we see none of the awkwardness that Emma had to have Frank's help to conquer when she met him at Randalls after his engagement was announced.